Spring Cloud Stream with Kafka - Lean Java Web Applications

Overview

This sample project demonstrates how to build real-time streaming applications using event-driven architecture, Spring Boot,Spring Cloud Stream, Apache Kafka and Lombok.

By the end of this tutorial you’ll have a simple Spring Boot based Greetings micro-service running that

takes a message from a REST api
writes it to a Kafka topic
reads it from the topic
outputs it to the console

Let’s get started!

Btw, you’ll find the source code here.

What is Spring Cloud Streaming?

Spring Cloud Stream is a framework built upon Spring Boot for building message-driven microservices.

What is Kafka?

Kafka is a popular high performant and horizontally scalable messaging platform originally developed by LinkedIn.

Installing Kafka

Download Kafka from here and untar it:

> tar -xzf kafka_2.11-1.0.0.tgz
> cd kafka_2.11-1.0.0

Start Zookeeper and Kafka

On Windows:

> bin\windows\zookeeper-server-start.bat config\zookeeper.properties
> bin\windows\kafka-server-start.bat config\server.properties

On Linux or Mac:

> bin/zookeeper-server-start.sh config/zookeeper.properties
> bin/kafka-server-start.sh config/server.properties

If Kafka is not running and fails to start after your computer wakes up from hibernation, delete the <TMP_DIR>/kafka-logsfolder and then start Kafka again.

What is Lombok?

Lombok is a java framework that automatically generates getters, setters, toString(), builders, loggers, etc. in the code.

Maven dependencies

Go to https://start.spring.io to create a maven project:

Add necessary dependencies: Spring Cloud Stream, Kafka, Devtools (for hot redeploys during development, optional), Actuator (for monitoring application, optional), Lombok (make sure to also have the Lombok plugin installed in your IDE)
Click the Generate Project button to download the project as a zip file
Extract zip file and import the maven project to your favourite IDE

Notice the maven dependencies in the pom.xml file:

  <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-actuator</artifactId>
  </dependency>
  <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-stream</artifactId>
  </dependency>
  <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-stream-kafka</artifactId>
  </dependency>
  <!-- Also install the Lombok plugin in your IDE -->
  <dependency>
      <groupId>org.projectlombok</groupId>
      <artifactId>lombok</artifactId>
      <optional>true</optional>
  </dependency>

  <!-- hot reload - press Ctrl+F9 in IntelliJ after a code change while application is running -->
  <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-devtools</artifactId>
      <optional>true</optional>
  </dependency>

… also the <dependencyManagement> section:

<dependencyManagement>
  <dependencies>
    <dependency>
      <!-- Import dependency management from Spring Boot -->
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-dependencies</artifactId>
      <version>${spring-boot.version}</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-stream-dependencies</artifactId>
      <version>${spring-cloud-stream.version}</version>
      <type>pom</type>
      <scope>import</scope>
    </dependency>
  </dependencies>
    </dependencyManagement>

… and the <repository> section:

<repository>
  <id>spring-milestones</id>
  <name>Spring Milestones</name>
  <url>http://repo.spring.io/libs-milestone</url>
  <snapshots>
    <enabled>false</enabled>
  </snapshots>
</repository>

Define the Kafka streams

package com.kaviddiss.streamkafka.stream;

import org.springframework.cloud.stream.annotation.Input;
import org.springframework.cloud.stream.annotation.Output;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.SubscribableChannel;  

public interface GreetingsStreams {
    String INPUT = "greetings-in";
    String OUTPUT = "greetings-out";

    @Input(INPUT)
    SubscribableChannel inboundGreetings();

    @Output(OUTPUT)
    MessageChannel outboundGreetings();
}

In order for our application to be able to communicate with Kafka, we’ll need to define an outbound stream to write messages to a Kafka topic, and an inbound stream to read messages from a Kafka topic.

Spring Cloud provides a convenient way to do this by simply creating an interface that defines a separate method for each stream.

The inboundGreetings() method defines the inbound stream to read from Kafka and outboundGreetings() method defines the outbound stream to write to Kafka.

During runtime Spring will create a java proxy based implementation of the GreetingsStreams interface that can be injected as a Spring Bean anywhere in the code to access our two streams.

Configure Spring Cloud Stream

Our next step is to configure Spring Cloud Stream to bind to our streams in the GreetingsStreams interface. This can be done by creating a @Configuration class com.kaviddiss.streamkafka.config.StreamsConfig with below code:

package com.kaviddiss.streamkafka.config;

import com.kaviddiss.streamkafka.stream.GreetingsStreams;
import org.springframework.cloud.stream.annotation.EnableBinding;

@EnableBinding(GreetingsStreams.class)
public class StreamsConfig {
}

Binding the streams is done using the @EnableBinding annotation where the GreatingsService interface is passed to.

Configuration properties for Kafka

By default, the configuration properties are stored in the src/main/resources/application.properties file.

However I prefer to use the YAML format as it’s less verbose and allows to keep both common and environment-specific properties in the same file.

For now, let’s rename application.properties to application.yaml and paste below config snippet into the file:

spring:
  cloud:
    stream:
      kafka:
        binder:
          brokers: localhost:9092
      bindings:
        greetings-in:
          destination: greetings
          contentType: application/json
        greetings-out:
          destination: greetings
          contentType: application/json

The above configuration properties configure the address of the Kafka server to connect to, and the Kafka topic we use for both the inbound and outbound streams in our code. They both must use the same Kafka topic!

The contentType properties tell Spring Cloud Stream to send/receive our message objects as Strings in the streams.

Create the message object

Create a simple com.kaviddiss.streamkafka.model.Greetings class with below code that will represent the message object we read from and write to the greetings Kafka topic:

package com.kaviddiss.streamkafka.model;

// lombok autogenerates getters, setters, toString() and a builder (see https://projectlombok.org/):
import lombok.Builder;
import lombok.Getter;
import lombok.Setter;
import lombok.ToString;

@Getter @Setter @ToString @Builder
public class Greetings {
    private long timestamp;
    private String message;
}

Notice how the class doesn’t have any getters and setters thanks to the Lombok annotations. The @ToString will generate a toString() method using the class’ fields and the @Builder annotation will allow us creating Greetings objects using fluent builder (see below).

Create service layer to write to Kafka

Let’s create the com.kaviddiss.streamkafka.service.GreetingsService class with below code that will write a Greetingsobject to the greetings Kafka topic:

package com.kaviddiss.streamkafka.service;

import com.kaviddiss.streamkafka.model.Greetings;
import com.kaviddiss.streamkafka.stream.GreetingsStreams;
import lombok.extern.slf4j.Slf4j;
import org.springframework.messaging.MessageChannel;
import org.springframework.messaging.MessageHeaders;
import org.springframework.messaging.support.MessageBuilder;
import org.springframework.stereotype.Service;
import org.springframework.util.MimeTypeUtils;

@Service
@Slf4j
public class GreetingsService {
    private final GreetingsStreams greetingsStreams;

    public GreetingsService(GreetingsStreams greetingsStreams) {
        this.greetingsStreams = greetingsStreams;
    }

    public void sendGreeting(final Greetings greetings) {
        log.info("Sending greetings {}", greetings);

        MessageChannel messageChannel = greetingsStreams.outboundGreetings();
        messageChannel.send(MessageBuilder
                .withPayload(greetings)
                .setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.APPLICATION_JSON)
                .build());
    }

The @Service annotation will configure this class as a Spring Bean and inject the GreetingsService dependency via the constructor.

The @Slf4j annotation will generate an SLF4J logger field that we can use for logging.

In the sendGreeting() method we use the injected GreetingsStream object to send a message represented by the Greetings object.

Create REST api

Now we’ll be creating a REST api endpoint that will trigger sending a message to Kafka using the GreetingsService Spring Bean:

package com.kaviddiss.streamkafka.web;

import com.kaviddiss.streamkafka.model.Greetings;
import com.kaviddiss.streamkafka.service.GreetingsService;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseStatus;
import org.springframework.web.bind.annotation.RestController; 

@RestController
public class GreetingsController {
    private final GreetingsService greetingsService;

    public GreetingsController(GreetingsService greetingsService) {
        this.greetingsService = greetingsService;
    }

    @GetMapping("/greetings")
    @ResponseStatus(HttpStatus.ACCEPTED)
    public void greetings(@RequestParam("message") String message) {
        Greetings greetings = Greetings.builder()
            .message(message)
            .timestamp(System.currentTimeMillis())
            .build();

        greetingsService.sendGreeting(greetings);
    }
}

The @RestController annotation tells Spring that this is a Controller bean (the C from MVC). The greetings() method defines an HTTP GET /greetings endpoint that takes a message request param and passes it to the sendGreeting() method in GreetingsService.

Listening on the greetings Kafka topic

Let’s create a com.kaviddiss.streamkafka.service.GreetingsListener class that will listen to messages on the greetingsKafka topic and log them on the console:

package com.kaviddiss.streamkafka.service;

import com.kaviddiss.streamkafka.model.Greetings;
import com.kaviddiss.streamkafka.stream.GreetingsStreams;
import lombok.extern.slf4j.Slf4j;
import org.springframework.cloud.stream.annotation.StreamListener;
import org.springframework.messaging.handler.annotation.Payload;
import org.springframework.stereotype.Component;

@Component
@Slf4j
public class GreetingsListener {
    @StreamListener(GreetingsStreams.INPUT)
    public void handleGreetings(@Payload Greetings greetings) {
        log.info("Received greetings: {}", greetings);
    }
}

The @Component annotation similarly to @Service and @RestController defines a Spring Bean.

GreetingsListener has a single method, handleGreetings() that will be invoked by Spring Cloud Stream with every new Greetings message object on the greetings Kafka topic. This is thanks to the @StreamListener annotation configured for the handleGreetings() method.

Running the application

The last piece of the puzzle is the com.kaviddiss.streamkafka.StreamKafkaApplication class that was auto-generated by the Spring Initializer:

package com.kaviddiss.streamkafka;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class StreamKafkaApplication {

    public static void main(String[] args) {
        SpringApplication.run(StreamKafkaApplication.class, args);
    }
}

No need to make any changes here. You can either run this class as a Java application from your IDE, or run the application from the command line using the Spring Boot maven plugin: